The types of storms that produce hail, usually supercellular, often have damaging straight-line winds. While out storm chasing, I've seen several gas station canopies collapse from strong winds - I'm struggling to think of a canopy solution that wouldn't have the risk of collapse.
Also, I'm imagining canopies getting in the way of 1) making inventory highly visible (both from the road and other parts of the lot) and 2) making it harder to pack cars really close together because you have to deal with all the canopy supports.
Just musings...
A canopy that isn't solid and allows the wind to pass through. A mesh or screen should do the job. The opening size can also be larger so perhaps even netting with openings upward of a few cm could work.
The forward flank downdraft or rear inflow jet of a linear convective system/bow echo can see winds of 50mph+ rather commonly. The more interesting part IMO, is the rear flank downdraft, because it is closely related to tornadogenesis and can also have winds up to 100mph. The RFD is co-located with the hail core in a supercell, so some hail will get caught up in this wind that exists even when there isn't a tornado.
Now terminal velocity of your average baseball-sized hail is roughly 100mph, but you can imagine with 100mph horizontal winds that there can be a strong non-vertical component to the hail in certain portions of the storm. I've personally been hit by smaller 1-2" hail that is traveling nearly horizontal, and I've seen evidence of larger hail impacting at the same angle.
It's a safe assumption that the larger the hail is the more damaging it is, but not all hail is created equal! Depending on how accretion happens and the warmth of the atmosphere, you can have very hard 1" hail that damages cars, and very soft 2.5" hail that doesn't damage cars. My personal rule of thumb is that you need to be wary of anything 1.75"/golf ball or larger.
Alright, that's not really a canopy, more like a shield or body armor.
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It wouldn't be hard to make some thick foam mats with straps sewn to them that can be rolled out and fastened to cars in under a minute each. Maybe toss in a couple plastic strips to make it unroll itself like the reverse of a slap koozie. Weather forecasting is pretty good. It's not like you don't have decent warning. You can cover everything out back and leave the cars that are on display out front uncovered until the last minute.
Tossing a mat over a high roof van or large pickup wouldn't be easy but the hordes of crossovers could be well protected.
OK, maybe it does almost sound feasible, but I wonder what someone with outdoor car dealership experience would say? Is this a practical idea? How often do you need to unroll those mats?
Maybe a few be stored, rolled up, under vehicles at the back of the lot that are less likely to be moved?
And the rest could be stored indoors. They shouldn't be heavy, so maybe they can be stashed in the ceiling, or somewhere high up out of sight?
You could even offer a deluxe version with a CO2 cartridge.
You made a good point, anyway. Foam could absorb water, and air, and get dirty, and smell bad.
I was musing at the time why someone on Amazon wasn't selling some kind of hail protection padded car cover.
I have tons of great ideas, but I never connect on them. Then I watch other people make a fortune off that same idea. Shrug...